Maintained by Robin Tecon, microbiologist and postdoctoral researcher at the Swiss Federal Institute of Technology Zürich. This blog is about bacteria (and other microbes) and the scientists who study them.

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Monday, March 19, 2012

Reporter bacteria to monitor arsenic concentration in groundwater

It is not widely known, but analytical measurements with bacterial bioreporters can compete with commercially available detection kits, inasmuch as the bioreporters' efficiency (sensitivity, detection limit, etc.) is often comparable - if not better - than the chemical-based systems. So why are bioreporters not more widespread? Well, the leap from the laboratory to the field is a difficult one, which demands a tight collaboration between fundamental research and engineering. In addition, economical and policy challenges need to be overcome. Today, most developed bioreporters have never left the lab.

In this study, the authors used a strain of E. coli that produces bioluminescence when it is exposed to arsenite and arsenate (oxidized forms of arsenic). Bacteria detect the poison thanks to a specific activator protein which binds arsenic and triggers the expression of specific genes. The result is the production of a set of proteins, including the bacterial luciferase, which makes the bacteria glow.

The measurement process is very simple: one milliliter of groundwater is added to a vial containing lyophilized bacteria; the vial is then shaken to mix the cells and the water together and it is incubated for 2 hours. The light coming out of the vial is directly measured using a portable luminometer, and the concentration of arsenic in the water is calculated from a standard curve obtained with known concentrations of arsenic. That's it!

So why should one want to measure arsenic concentration? And why should one expect to find this poison in the water in the first place? The fact is that arsenic (an element of Mendeleev's table) is naturally present in many minerals in the environment. Thus, it is owing to geological conditions that groundwater can be contaminated with elevated concentration of arsenic. Drinking this poisonous water is not directly fatal, but can lead to severe health disorders, even to cancer, and it can take years before the effects of the poison appear. Arsenic contamination of groundwater is a world problem, but it is particularly acute in Bangladesh, a country of 150 million inhabitants where approximately one inhabitant out of three is exposed to arsenic.

The situation in Bangladesh is indeed very sad: forty years ago, when the country obtained its independence, a vast campaign was initiated with the help of the UN to drill water wells and therefore solve the persistent problems of water insalubrity that caused countless cases of water-borne disease. The campaign was a success and the population gained access to clean water. However, because of Bangladesh's geological conditions, many wells turned out to be contaminated with arsenic, threatening the population with slow poisoning. Between Scylla and Charybdis…

Without analysis, it is impossible to tell the clean from the contaminated groundwater. Often, contaminated and clean wells coexist within the same area (a village, for instance), and it would be most advisable to know the arsenic content in each well. But available testing kits are expensive and necessitate the manipulation of toxic compounds. By contrast, reporter bacteria are relatively cheap to produce, require less material and chemicals than the commercial kits and comparatively permit to test more samples within the same amount of time. Whereas 50 milliliters of groundwater are needed in the chemical test, only one milliliter is used with the bioreporters. A nice account (including video) of this field testing can be found on the website of Deutsche Welle, a german media which followed the team of scientists in Bangladesh